1. Field of the Invention
The present invention relates to packaging for large scale integrated electronic circuits, and more particularly to terminals on chips in the form of bumps adapted to bonding of a lead to connect the chip in an electronic system.
2. Description of Related Art
Low cost packaging of electronic chips requires that chips be manufactured in large quantities at low cost. Tape Automated Bonding (TAB) is a way of reducing labor costs in bonding chips to packages. Most commonly, TAB packaging involves the provision of a 10-25 .mu.m tall metallic bump, usually composed of gold on a bare aluminum I/O pad on the surface of the chip. The bump has served as a bonding platform extending very high above the top surface of the chip thereby providing a pedestal above the surrounding insulation and allowing connections to the underlying metallurgy. The tall bump also protects the pad below from corrosion by covering it and isolating it from the environment.
In general the gold bumps or their equivalent have been applied to the chips by means of electroplating or similar wet chemical additive processes of deposition of metal onto the chip. These wet chemical processes require the immersion of the entire chip wafer into the electroplating bath, exposing the wafer to all of the chemicals in the bath. The removal of the common plating electrode (necessary for the plating operation) must be further accomplished by another wet chemical etching step, ion-beam etching or the equivalent.
Virtually all of the papers and patents on semiconductor attachment techniques use aluminum in making ohmic contact to the semiconductor but have not used aluminum in the bulk material of solid bumps. Aluminum is commonly used as a thin film wiring level material. An early article, Noubel et al "Metallurgy Including a Chromium Slice", IBM Technical Disclosure Bulletin p 769 Dec. 1968) uses aluminum as an ohmic contact as a matter of course. Connections to the semiconductor chip are made by a thin film interface scheme including one or more thin film layers, typically up to 2 .mu.m thick, followed by a thick (typically 20 .mu.m or more) layer of either a soft low melting point metal for soldering or reflow bonding (U.S. Pat. No. 3,886,585 of Konantz et al -discussed below in detail) or a hard metal for thermocompression bonding (Harris U.S. Pat. No. 4,005,472). One patent (Harris U.S. Pat. No. 4,042,954, Aug. 16, 1977-discussed below) uses a thin film of aluminum/chromium as an additional conduction layer, 2 .mu.m thick, below a bump of nickel and copper.
It appears that aluminum is not thought of in terms of a bulk material for contacts even though it is used throughout the industry as a thin film metal for contact to semiconductors, for wiring chips, and even, as in Harris U.S. Pat. No. 4,042,954 as an underlying thin film layer for hard bumps. Apparently the thought is that aluminum is unsuited to serve as a pedestal for thermocompression bonding. All of the prior art makes use of hard metals for thermocompression bonding pedestals, Harris U.S. Pat. No. 4,042,954 uses nickel and copper, Harris U.S. Pat. No. 4,005,472 uses copper, Meyer et al, "Metallurgy of Ti-W/Au/Cu system for TAB assembly" J. Vac. Sci. Technol A 3(3) pp 772-776 (May/Jun. 1985) and R. F. Unger, C. Burns, and J. Kanz "Bumped Tape Automated Bonding (BTAB) Applications" describe electroplated gold bumps. (Gold is normally rather hard in the plated condition.) Aluminum may be thought to be too soft for use as a bulk material. We have demonstrated that the softness of the aluminum makes possible good bonds while at the same time protecting the underlying structures from excessive force during bonding.